This invention relates to photoflash lamps and, more particularly, to photoflash lamps characterized by a duration of light emission at useful light levels many times longer than that of the more popular types of presently available commercial flashlamps.
Generally speaking, the present art of making photoflash lamps is concerned primarily with photographic flashlamps which have a high intensity light peak or a certain level of light intensity for a maximum duration in the millisecond range. Such a light source usually comprises an hermetically sealed envelope, provided with a protective coating, and filled with a combustion-supporting gas and a combustible, usually in the form of shredded foil, which upon ignition, burns with a brilliant, fast flash. The ignition system usually comprises a pair of lead-in wires, a coating of ignition paste on the inner ends of the lead-in wires, and a fine tunsten filament attached to and supported by the lead-in wires near the inner ends thereof. Other currently used ignition systems for photoflash lamps include percussively activated primer tubes and high voltage activated primer bridge structures.
Although this type of photographic flashlamp has proved to be quite satisfactory for the more conventional uses, it has been found that for certain applications, such as motion picture, and especially high-speed motion picture, photography for example, a light source of useful light levels characterized by a duration of light emission in the order of seconds is highly desirable. Such lamps are particularly useful in obscure areas where it is not feasible or practical to run power cables. These uses include spelunking, crashing of cars to observe operation of safety equipment, etc. Another use has been in the space program.
Long-duration lamps have been made since the late '50's. For example, U.S. Pat. No. 2,955,447 Fink et al describes such a lamp containing a magnesium strip and a gas filling consisting of oxygen and between 2% to about 12% of nitrogen by volume. Subsequently, U.S. Pat. No. 3,002,367 Fink issued and described an improved support structure for the foil strip in a long-duration lamp. In this latter Fink patent, the magnesium foil strip was corrugated into a W-shaped configuration and was provided with a plurality of apertures. The foil strip is supported over a filament ignition structure, and the edge portion of the foil strip adjacent to the ignition structure is coated with primer material to facilitate firing of the strip. If this primer coating is disposed on a plane foil surface, a problem has been experienced in that the primer tends to come loose either before or after the sealing-in operation. Accordingly, a row of straight embossments were provided along the edge of the foil where the primer is applied; the resulting added surface area causes the primer mixture to adhere better to the magnesium. This technique of embossing the lower edge of the magnesium strip for improved primer adhesion has been used for several years but is not shown in the above-mentioned Fink patent.
Although the long-duration photoflash lamp described in the Fink U.S. Pat. No. 3,002,367 provides excellent performance for its intended applications, the time-light curve of the lamp is characterized by two or three peaks, and the beginning and trailing portions of the curve are quite rounded. It would be advantageous to have a lamp with a higher light level while still maintaining a long-time duration. Better quality photographs can be obtained if the combustion proceeds more evenly.
A U.S. Pat. No. 3,199,316 of Cressman describes a long-duration lamp which is claimed to emit at least 10,000 lumens for a time interval in the order of 7 to 10 seconds. The lamp contains a gas fill of approximately 33% Argon and 67% oxygen, and a comparatively long strip of magnesium foil is supported longitudinally within the lamp bulb. The magnesium strip in Cressman is provided with an array of combustion regulating tabs which are claimed to keep the foil from curling while burning.
In contrast to Cressman, the lamp defined by the above-mentioned Fink and Fink et al patents employs a different fill gas composition and a much smaller and more compact foil configuration. The Fink patent uses a W-shaped corrugated configuration to space the foil away from the sides of the metal supporting frame, as proximity of the frame to the magnesium strip dampens the burning rate. In contrast, Cressman defines a planar strip. The arrangement of combustion-regulating tabs employed on this planar strip of Cressman has not been found suitable for the W-shaped foil configuration and gas fill composition of the lamp described in the Fink patent.